Using molecular, biochemical and structural approaches, over the past many years, we have contributed to defining how specific human BER proteins recognize and process target lesions, as well as coordinate with other components of the pathway. This research has centered largely on apurinic/apyrimidinic endonuclease 1 (APE1), the major mammalian protein for repairing abasic sites in DNA, and x-ray cross-complementing 1 (XRCC1), a non-enzymatic scaffold protein that facilitates the efficient execution of single-strand break (SSB) repair. Our most recent work has found that (i) the protein activation-induced cytosine deaminase (AID) generates uracil DNA intermediates in immunoglobulin gene variable and switch regions as part of the mechanism to create antibody diversity; (ii) XRCC1 influences the processes of somatic hypermutation and class switch recombination, two key elements of the immune response and antibody generation, by modulating different mutagenic pathways; (iii) XRCC1 coordinates disparate responses and multi-protein repair complexes that are dependent on the nature and context of the DNA damage; (iv) the different regions of XRCC1 play distinct roles in coordinating repair complex assembly and the population variants Arg280His and Arg399Gln exhibit reduced stability at DNA damage foci, suggesting that they may represent disease susceptibility factors; (v) APE1 can endonucleolytically cleave RNA substrates in its role to cleanse the transcript pool and regulate gene expression via a reaction mechanism that is similar but distinct from that used on AP-DNA; and (vi) the DNA repair function of APE1 is regulated by a novel post-translational modification, S-glutathionylation. APE1 is the major mammalian enzyme responsible for the repair of abasic sites in DNA, and has functions in SSB repair and other cellular processes, including transcriptional regulation. We are in the process of establishing genetically modified cell lines to dissect out the precise contribution of each proposed function of APE1 (i.e. its nuclease activity, redox regulatory role, etc.) in cell growth/viability, genome maintenance, and protection against DNA-damaging agents. The underlying hypothesis is that APE1, and BER more generally, plays a critical role in dictating cellular responsiveness to genotoxic insults and susceptibility to disease manifestation, particularly in the face of certain environmental exposure(s). Defining which of the many reported functions of APE1 are critical to normal cellular activity is a key step towards understanding the potential relationship of the protein to the aging process and disease risk. DNA interstrand crosslinks (ICLs) can arise from reactions with endogenous chemicals, such as the lipid peroxidation product malondialdehyde, as well as from exposure to a variety of clinical anti-cancer agents, such as bifunctional alkylators. Recent studies suggest a role for BER in the processing of these highly toxic DNA intermediates, which are absolute blocks to transcription and replication, and thus, lethal adducts. We are performing experiments using developed strategies, e.g. laser microirradiation coupled with high resolution confocal microscopy, to define the molecular involvement of specific BER factors in ICL removal. Elucidating the roles of BER proteins in ICL resolution may reveal new targets relevant to anti-cancer treatment paradigms.